1. Field of the Invention
This invention relates to a steel-pipe coating process and an apparatus therefor, in which molten resin is extruded through a cross head die connected to an extruder, into a tubular shape, in a manner to cover the outer surface of a steel pipe, and more particularly to a process and an apparatus in which the amount of resin being extruded through a cylindrical outlet in the cross head die around the surface of the steel pipe may be intermittently varied, thereby forming joints or nodes in the resin coating covering the surface of the steel pipe.
2. Description of the prior art
Hitherto,many attempts have been proposed for forming joints or nodes, i.e., annular ribs on a resin coating covering the surface of a steel pipe, thus presenting a bamboo-like appearance. These attempts are characterized by:
(a) a process for coating the surface of a steel pipe by using a cross head die in an extruder, in which the feeding of a steel pipe is intermittently interrupted, so that resin being fed from the cross head die is intermittently built up in specific positions around the surface of the steel pipe, thereby forming annular ribs therearound;
(b) a process, in which an extruder and a cross head die are of one body and the extruder and the cross head die are jointly reciprocated within a given stroke and the relative speed of the extruder, as well as the cross head die, to the steel pipe being fed is intermittently varied so as to form annular ribs around the surface of the pipe; and
(c) a process, in which there are provided a main extruder for extruding resin for use in coating and an auxiliary extruder for extruding resin to form ribs.
However, according to process (a), the feeding of a steel pipe is intermittently interrupted, thus failing to form a coated steel pipe continuously so that process (a) is locking in efficiency and not adapted for mass production.
According to process (b), an extruder and a cross head die are jointly reciprocated, thus dictating the use of a large scale apparatus with the accompanying increase in power required therefor as well as in installation cost. According to the process (c), two extruders are required resulting in an increase in scale of the apparatus and increased installation cost.